1. Field of the Invention
The invention relates to a method of encoding/decoding effectively a fading image and a dissolve image and an apparatus therefor.
2. Description of the Related Art
In a video encoding standard scheme such as ITU-TH.261, H.263, ISO/IECMPEG-2, and MPEG-4, a motion compensated predictive interframe encoding is used as one of encoding modes. As a predictive model in the motion compensated predictive interframe encoding is adopted a model indicating the most high predictive efficiency when luminosity does not vary in a time axis. In the case of the fading image that luminosity of the image varies, for example, in the case of fading-in from a black image to a normal image, a method of performing a prediction adequately according to a change of luminosity of the image is not known. Consequently, there is a problem to need the large number of encoded bits for the purpose of maintaining an image quality in a fading image.
For example, U.S. Pat. No. 3,166,716 discloses a technique of coping with the problem by detecting a fading image area and changing an allocation of the number of encoded bits thereto. Concretely, in the case of the fade-out image, the large number of encoded bits are allocated to an beginning part of the fade-out that brightness varies. The last part of the fade-out usually reduces allocation of the number of encoded bits since it normally becomes a monochrome image resulting in making the encoding easy. With such a way, a total image quality is improved without increasing the total number of encoded bits.
On the other hand, U.S. Pat. No. 2,938,412 discloses an encoding system that deals with the above problem in a fading image by compensating for a reference image according to two parameters of an amount of luminance change and an amount of contrast change.
Thomas Wiegand and Berand Girod, “Multi-frame motion-compensated prediction for video transmission”, Kluwer Academic Publishers 2001, provides an encoding system based on a plurality of frame buffers. This system intends to improve a predictive efficiency by generating a predictive image selectively from a plurality of reference frames saved in the frame buffers.
However, the system of U.S. Pat. No. 3,166,716 improves an image quality without increasing the total number of encoded bits in encoding a fading image by detecting a fading image area, and changing an allocation of the number of encoded bits. For this reason, there is the advantage that the encoding can be realized within the framework of an existing encoding system. However, since the prediction efficiency is not essentially improved, a notable improvement of the encoding efficiency cannot be expected.
On the other hand, the system of U.S. Pat. No. 2,938,412 has a merit that the predictive efficiency on a fading image improves. However, the predictive efficiency to be enough for so-called a dissolve image (referred to as a cross fade image) which an image gradually varies from an image to another image cannot be obtained.
The system of Thomas Wiegand and Berand Girod cannot sufficiently deal with a fading image and a dissolve image, and cannot improve a predictive efficiency even if a plurality of reference frames are prepared.
According to the prior art as described above, the large number of encoded bits are required for the fading image and dissolve image to be encoded with high image quality. There is a problem that improvement of the encoding efficiency cannot be expected.